Referring to FIG. 1, a conventional down-conversion mixer includes a first single-ended to differential converting circuit 11, a second single-ended to differential converting circuit 12, a transconductance circuit 13, a mixing circuit 14 and a buffering circuit 15.
The first single-ended to differential converting circuit 11 converts a single-ended oscillatory voltage signal into a differential oscillatory voltage signal pair. The second single-ended to differential converting circuit 12 converts a single-ended input voltage signal of radio frequency into a differential input voltage signal pair. The transconductance circuit 13 includes four transistors 131-134 and an inductor 135, and converts the differential input voltage signal pair into a differential input current signal pair. The mixing circuit 14 includes two resistors 141, 142 and four transistors 143-146, and mixes the differential input current signal pair with the differential oscillatory voltage signal pair to generate a differential mixed voltage signal pair of intermediate frequency. The buffering circuit 15 buffers the differential mixed voltage signal pair to generate a differential output voltage signal pair.
When a frequency of the differential mixed voltage signal pair is low, a conversion gain (CG) of a combination of the transconductance circuit 13 and the mixing circuit 14 can be expressed by the following equation:
                              C          ⁢                                          ⁢          G                ≈                                            2              π                        ·            sin                    ⁢                                          ⁢                                    c              ⁡                              (                                  π                  ·                                      Δ                    /                                          T                      OS                                                                      )                                      ·                          (                                                g                                      m                    ⁢                                                                                  ⁢                    131                    ,                    132                                                  +                                  g                                      m                    ⁢                                                                                  ⁢                    133                    ,                    134                                                              )                        ·                          R              L                                                          Equation        ⁢                                  ⁢        1            where gm131,132 denotes a transconductance of each of the transistors 131, 132, gm133,134 denotes a transconductance of each of the transistors 133, 134, RL denotes a resistance of each of the resistors 141, 142, TOS denotes a period of the differential oscillatory voltage signal pair, and Δ denotes a half of the fraction of the period in which the transistors 143-146 all conduct.
Since all of the parameters in Equation 1 are fixed, both the CG of the combination of the transconductance circuit 13 and the mixing circuit 14 and a CG of the conventional down-conversion mixer are fixed. In addition, the CG of the conventional down-conversion mixer is low, and a noise figure of the conventional down-conversion mixer is high.